A scanning sonar has been used for detecting underwater objects. The scanning sonar includes an approximate cylindrical transducer to detect an object in all directions around the transducer as shown in FIG. 1. Each vibrator of the transducer is driven to form an ultrasonic transmitting beam in all directions therearound. Signals for driving each of transducer elements (vibrator) are controlled to provide a transducer element array in each row with an appropriate delay time or phase difference so that a transmitting beam, which has specific umbrella type tilt angle keeping a directivity in vertical direction being within the specific angle, can be formed. A detection over all directions can be made by turning a direction of a receiving beam sequentially by switching over a combination of specific number of transducer element arrays disposed cylindrically which forms receiving beam in a specific direction.
In a conventional scanning sonar, there are a horizontal mode where a detection with a specific tilt angle is made in all surrounding directions and a vertical mode where a detection with a larger tilt angle is made for searching a sector-form cross section in a vertical direction. The horizontal mode is mainly used for detecting objects within a specific range around the boat. The vertical mode is mainly used for detecting objects locating within an area with specific width approximately directly below the boat.
Normally either the horizontal mode or the vertical mode is chosen according to the purpose of detection. In some cases, however, detecting objects both within a specific range around the boat and within an area with specific width approximately directly below the boat is desired at the same time. For that purpose, there is a horizontal-vertical multiple mode which displays detected two images from both the horizontal mode and the vertical mode on a single screen.
In the detecting method using a conventional scanning sonar, detected image of specific cross section is formed by transmitting an ultrasonic pulse of a single frequency toward the area to be detected and receiving the pulse reflected from the object and others sequentially at every specific direction. Consequently a control of forming both transmitting beam and receiving beam for the horizontal mode for obtaining the detected image in horizontal mode, and a control of forming both transmitting beam and receiving beam for the vertical mode for obtaining the detected image in vertical mode must be performed in turn.
This causes to make it slow to renew the detected image, in other words images which shows quick change cannot be detected. Also when the two detected images from the horizontal mode and the vertical mode are displayed on the same screen, the time those two images are detected is different (neither the same nor close to each other), which leads to difficulty in reading and understanding the relation therebetween while comparing the two images.
The problem described above arises not only in the above case, but also in the case where both objects in lateral direction, i.e. direction horizontal or direction tilted from horizontal by specific angle and objects in longitudinal direction, i.e. direction vertical or direction tilted from vertical by specific angle are tried to be detected in the same time or within a short period of time.
It is also important to avoid interference between two kinds of received signals when transmitting/receiving ultrasonic signal for detecting objects and receiving ultrasonic signal which has been transmitted for another purpose are performed concurrently. For example, at the time of transmitting/receiving ultrasonic signal for fish-finding or tidal current measuring, detecting operation as a scanning sonar is temporally suspended. This makes, however, the detecting operation intermittent and it difficult to perform detection in a short time period.